1. Field of the Invention
This invention relates to multi-antenna systems for communication systems and more particularly to selection of antenna within a multi-antenna system for use in communication systems.
2. Description of Related Art
A conventional wireless communication system facilitates two-way communication between a plurality of subscriber units and a wireless network infrastructure. Typically the wireless infrastructure is fixed and the subscriber units can be fixed or mobile, or a combination of both fixed and mobile. A typical environment where a wireless network operates includes many structures and obstructions that can block or reflect the wireless signal as it propagates between the subscriber units and the infrastructure. For example, as the signal propagates it may be reflected off of walls, people or other obstacles within the wireless environment. As the signal propagates, and is reflected, it may arrive at a receiver from a different angle than a direct path between the transmitter and receiver.
The wireless environment also changes over time. For example, in a wireless LAN, the obstacles within the environment move, such as people walking, or furniture or equipment being moved. In addition, if the wireless receiver or transmitter are mobile, their movement also changes the location of the obstacle relative to the receiver or transmitter. As the wireless environment changes so does the path that a signal takes as it propagates through the environment. The change in propagation path means that it is unknown what direction the signal will arrive at the receiver from a transmitter.
FIG. 1 is a diagram of an example wireless network 100. As shown in FIG. 1, the wireless network includes a transmitter 102 and a receiver 104. In a typical environment where a wireless network 100 operates there are also obstacles 106. In an outdoor environment, examples of obstacles 106 include people, buildings, cars, trees, etc. In an indoor environment some examples of obstacles 106 include walls, people, furniture, equipment, people, etc. As noted above, the environment also changes over time and the obstacles 106, transmitter 102, or receiver 104, move in relation to each other.
As illustrated in FIG. 1, as the radio signal propagates between from the transmitter 102 and is received at an antenna 120 of the receiver 104 it can be blocked, and reflected by the obstacles 106 in the surrounding environment. The example illustrated in FIG. 1 may be referred to as a non-line of sight (NLOS) propagation environment because the direct line-of-sight (LOS) path 110 is blocked by one of the obstacles 106. Likewise other instances of the signal propagate through the environment, being reflected off of obstacles and traveling various different paths. Four examples of these “multipath” signals 110, 112, 114, and 116 are illustrated in FIG. 1. As shown in the example of FIG. 1, the multipath signals can arrive at the receiver 104 from different directions.
To account for this unknown direction of the receive signal, many wireless devices use an omni direction antenna. While an omni directional antenna can receive signals from any direction, the received signal is not optimal because there is no antenna gain. A directional antenna has antenna gain over an omni antenna, but because it is directional it can only receive signals from a particular direction. However, because the wireless environment may be changing a fixed directional antenna may not provide adequate performance.
A technique used to enhance the performance of directional antennas is an antenna switching system. A typical antenna switching system includes multiple antennas, or antenna elements, arranged in a pattern that enables reception of a signal from any direction. With the antenna switching system, individual antenna elements may be selected such that the antenna “pointed” in the direction of the received signal is selected for use.
Therefore, there is a need for methods and apparatus to optimize antenna selection in a multi-antenna system.